The 139Ce absorption-edge transmission technique will be further developed to permit assay of stable iodine concentrations to within plus or minus 10 micrograms/ml in in vitro samples. Measuring times will be about 15 minutes or less and required sample volumes not more than 10 ml, probably 3-6 ml. Standard NaI(T lambda) detectors and counting electronics will be used for transmission measurements of characteristic x-rays of lanthanum emitted by the 139Ce source. The use of 139Ce or visualizing stable thyroidal iodine with a multi-wire proportional counter will also be investigated.